A substantial number of aqueous streams must be treated to meet governmental laws and regulations to certify them for drinking purposes or for release into the environment. Non-limiting examples of such aqueous streams include: those emanating from municipal water supplies; those waste water streams resulting from various chemical, petrochemical, and refining processes; and, those resulting from various other industries such as the pulp and paper industry. Contaminated ground water streams must also be treated depending on their intended use. Such aqueous streams typically contain one or more impurity, such as suspended matter, dissolved organic constituents, dissolved mineral matter, and microorganisms. The pulp and paper industry is faced with an additional problem of having to reduce color from its waste water streams. The color, which usually develops when wood is pulped, is derived primarily from the non-carbohydrate constituents of the wood, such as lignin. Lignin and certain extraneous components contain sites that can, under certain conditions, be readily converted to colored (chromophoric) groups. These constituents are generally chlorinated organic compounds which vary in molecular weight. The chlorinated lignins are typically high molecular weight molecules that tend to be stable against biodegradation. Therefore, it is desirable to break these high molecular species into smaller components which will be more susceptible to biodegradation.
Various methods have been developed over the years for purifying aqueous streams, including methods for removing or destroying color components. Early methods of treatment involved the use of aeration with a gas having a relatively high oxygen content, over a period of several days, to biodegrade various bacterial and organic substances. This typically resulted in the waste stream being environmentally safe except for the presence of color components which are not easily destroyed by aeration. Before the introduction of strict environmental laws and regulations, such streams, after aeration, could merely be fed into a moving river or stream, where their color would be diluted. The drought and resultant low flow conditions of receiving streams in the United States in the 1980's has caused increased concern about the environmental impact such colored waste streams have when released into rivers, streams, etc. Consequently, there is a substantial need to further purify these streams to a greater degree, at reasonable cost, so they will have little, if any, negative impact when released into the environment.
Other conventional methods for purifying aqueous streams include the addition of lime, activated carbon, or both, to precipitate, and adsorb colored bodies. In the case of lime, the resulting precipitate is typically gelatinous in nature, low in solids, difficult to separate, and is extremely resistant to dewatering by accepted methods. This presents a disposal problem of its own. Furthermore, much of the lime is lost during the process, adding greatly to the economics of the process itself. Activated carbon, which removes color components by adsorption, has also been used, but has proved to be uneconomical. That is, like the use of lime, the use of activated carbon results in the disadvantages of high cost and the continuous need for solids disposal or regeneration of reactant.
Still other conventional methods for treating waste water streams include the use of oxidizing agents, including oxygen, ozone, air, peroxides, permanganates, chromates, metal oxides, mineral acids, and organic oxidizing agents, such as peracetic acid. Further, U.S. Pat. No. 3,737,374 teaches the use of ozone to reduce the molecular weight of the lignins to less than about 10,000, thereby reducing its color intensity. These lower molecular weight species are also more susceptible to biodegradation because they are more easily digested by microorganisms. In fact, ozone is becoming the oxidizer of choice for purifying these streams.
While the above methods have met with various degrees of success in clarifying waste water streams which contain color bodies, there still remains a need in the art for ever more efficient processes for clarifying such streams.